You will always use "Je parle français / est-ce que tu parles français?".
If you put "le", it means you're talking about studies.
For instance, a child would say "j'aime les math(ématique)s mais je déteste le français, la prof(esseure) est méchante." (I like math but I hate french, the teacher is mean.)
PS: However, if you ask "Parlez-vous le français?", people will still understand you :)